Field of the Invention
The invention relates to optical metrology and related systems. In particular, it concerns in-line measurements of materials being processed in a roll-to-roll operation.
Description of the Prior Art
There is a need for in-line metrology for materials going through a roll-to-roll processing operation. The need exists both for raw substrates (such as for quality control of substrate materials by device manufacturers and quality measurements by the substrate manufacturers) and also for quality control of actual devices, both during and after processing for manufacture. Processing typically involves the deposition of uniform layers of material across the entire width of a plastic web substrate, or the deposition of traces, transistors, or other features, to create electronic circuits on flexible plastic substrates in an increasing number of applications, including displays, biomedical devices, smart apparel, and advanced sensors. Such flexible webs may be up to one meter in width and move at speeds greater than one meter per minute; therefore, real-time measurements of web features are difficult to carry out.
In order to achieve effective quality control during such processing, 3D metrology is essential because several major failure modes of plastic electronic devices are associated with parameters that require precise height measurements. Roughness has been found to be a major contributor to transistor failure in flexible circuits; therefore, it needs to be tightly measured and controlled. While very smooth substrates exist, they are very expensive. Thus, manufacturers often utilize lower-quality materials and either coat them to smooth them out, or measure the roughness and exclude very rough areas from further processing.
Regardless of quality, all plastic substrates unavoidably carry some defect within them. The slope of those defects can cause cracking—for example, if a conductive trace, transistor, or other element is deposited across a highly sloped feature, it is very likely to crack. However, if the defect is shallow, it is not usually a problem and there is no need to exclude it from further processing. Thus, 2D measurements of defects do not provide sufficient information for good quality-control decisions by manufacturers about whether or not to include the defect in the product area. Because of its high vertical and transverse resolution, large field-of-view, extremely fast measurement times, and nanometer precision, 3D optical metrology is particularly suited for in-situ roll-to-roll measurement of the height and slope of such defects in real time during the deposition process.
Existing interferometric inspection tools measure only small-areas and are suitable mainly for laboratory-bench work. With a typical lateral resolution of about 2 μm, such systems can measure areas less than 1 square mm in the span of several seconds, while also requiring vibration isolation. Current in-line methods are machine-vision-based with limited lateral resolution (˜100 μm) and often are incapable of measuring roughness or quantifying heights of defects. In addition, when testing transparent substrates, such in-line techniques often also suffer from reflections from the back side of the film, which may be only 25 μm thick, or from the roller mechanisms over which the film is traveling. In order to measure surface roughness in the range of 1 nm rms, the metrology tool must have micrometer-level lateral resolution and be capable of measuring flexible samples despite effects that can vary the position of the substrate relative to the tool.
Thus, any in-line metrology solution must deal with several major challenges. The first is that the substrate material, being flexible, flutters as it travels across rollers. This means that it moves up and down in relation to the roller, thus going in and out of focus of a stationary optical system that is looking at it. Because of this problem, in order to account for this flutter problem, existing metrology systems typically look at the substrate while in continuous contact with the substrate. This is very helpful, but it does not entirely solve the problem because of the potential damage to the substrate due to its contact with the measurement system while in motion over the roller.
Another challenge lies in the rollers themselves. They inherently have some amount of runout; that is, the amount by which the top of a roller will move up and down during a rotation around its axis. A typical runout is on the order of tens to hundreds of micrometers. Therefore, if an optical instrument is focused on the top of the roller, any significant amount of runout will also cause the optical image to be out of focus. Both web/substrate flutter and roller runout mean not only that a certain amount of focus variation must be tolerated by the optical system, but also that it be vibration-immune to avoid errors caused by the motion of the web and/or the roller, as well as by the overall machine vibration due to vacuum pumps, multiple motors, and other mechanical devices used in the production of flexible circuits.
Furthermore, because in-line optical metrology systems on roll-to-roll operations typically image onto the web while is passes by on the roller, the curvature of the roller means that there is a focal distance variation across the field of view of the objective (for example, if the metrology system is focused on the top of the roller, with the top in the center of the field of view, the edges will be slightly out of focus due to the roller's curvature). In view of the foregoing, any in-line instrument must account for web flutter and roller runout, it must be substantially vibration-immune, and it must account for the curvature of the rollers. Additionally, end users often require that the optical system contain minimal moving parts, so as to minimize incidents of failure. Finally, many stages of existing optical systems, such as those used for focusing, utilize vacuum-compatible components that are undesirable because of their cost. Therefore, a 3D optical system that overcame these problems would represent a valuable step forward in the art. This invention is directed at providing such a system.